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MMF space harmonics and slot openings produce considerable rotor losses in permanent magnet (PM) machines, especially if fractional-slot windings are adopted. This paper aims to measure the rotor losses of a 12-slot 10-pole axial flux phase modulation machine. Both MMF space harmonics and slot openings are considered. The prototype is an axial flux machine with open slots, equipped with three different rotor disks: a disk without PMs, a disk with energized PMs, and a disk with de-energized PMs. A three-layer analytical model is used to interpret the experimental results, e.g., to split the losses in the different parts of the rotor.